Black Hole
May contain traces of nut
You were right, I was worrying unnecessarily! I just used the existing "share" and bingo, it works! I do need to have a USB stick plugged in though.
The virtual-disk2 package overcomes the need for a real physical USB drive to be plugged in (the Humax software has to be poked by one or the other, in order to "see" our virtualised USB mounts).
However, it is reported that virtual-disk2 causes the Humax to display the "what do you want to do with this USB drive" icons every boot. Some people have reported this happens every time with a real USB drive plugged in too, but I have never seen it so I am interested to know how common this is and why it happens. I only see the USB menu when a drive is plugged in after the Humax has booted (having not been plugged in during boot).